Stabilized photographic silver halide emulsions



United States Patent ()7 3,128,186 STABILIZED PHOTUGRAPHKC SILVER HALHDE EMULSEQNS Leo D. Corhen, Gal; Park, Mich, and Ernst Aibert Steigmama, Winchester, Mass, assignors to General Foods Corporation, White Plains, N.Y., a corporaticn of Delaware No Drawing. Filed .lnly 15, 1960, Ser. No. 42,967 8 Claims. (Cl. 961d9) This invention relates to fog-inhibiting agents and stabilizers for photographic emulsions and to photographic emulsions containing them.

As is well known to those skilled in the art, there is a wide variety of materials which may be employed in the preparation of photographic emulsions. For example, it is possible to use as the dispersing or binding agent, any of a long list of materials including synthetic materials such as collodion, polyvinyl plastics, or others, in addition to the more commonly used on-synthetics most common of which is gelatin. Those in the photographic field have long appreciated that the photochemical properties of these materials and of the emulsions prepared therefrom must be determined empirically. It is impossible to tell, a priori, the handling properties, the sensitivity, the speed, the susceptibility to fogging, etc., of these materials.

There are, however, certain generalizations which can be made which clearly distinguish gelatin on the one hand from the so-called synthetic binding agents on the other hand. First, with respect to overall properties, gelatin is so superior to all of these other synthetic materials that its properties are substantially unique to the extent that generalizations which relate to the synthetic materials cannot be carried over into gelatin technology. More specifically, however, it is well known to those skilled in the art that silver halide emulsions prepared from synthetic binding agents or suspending agents are characterized by a substantial freedom from fogging tendencies and a very low sensitivity. This may be due, in part at least, to the fact that gelatin is obtained from a crude raw material and may contain numerous impurities therein, whereas the so-called synthetic materials may approach 100% chemical purity. Accordingly in dealing with synthetics, it has been customary to attempt to improve the sensitivity in order to permit use of these materials, although insofar as fogging tendencies are concerned, they have been eminently satisfactory, i.e. they are not subject to chemical fogging.

It is well known that photographic emulsions on storage tend to lose sensitivity and to become spontaneously developable without exposure to light. During development there is normally a detectable amount of the silver salt reduced in the areas where no exposure was given; this is commonly called fog, and sometimes called chemical fog to distinguish it from the effects of accidental exposure to radiation; in this invention, we are concerned with chemical fog.

Chemical fog depends both on the emulsion and the conditions of development; for a given emulsion it increases with the degree of development. With constant development conditions, it tends to increase with time, temperature, and relative humidity of storage conditions; it is common practice to make accelerated tests of the stability of photographic emulsions by storage at increased temperature and/or humidity. It is, of course, desirable to have emulsions as stable as possible under the conditions of high temperature and humidity which may occur in tropical climates, for example. Chemical fog usually appears over the whole area of the sensitive coating, but when severe, it frequently is non-uniform. Chemical fog may also be caused by exposure to chemicals, for

3,128,186 Patented Apr. 7, 1964 ice example, hydrogen sulfide and other reactive sulfur compounds, hydrogen peroxide vapor, and strongly reducing materials. While antifoggants and stabilizers may protect, to some extent, against such defects, it is normally understood that an antifoggant protects against spontaneous growth of fog during prolonged storage or storage at hi h temperatures and humidities, or during development to maximum contract and speed, or both.

It is, accordingly, an object of this invention to provide a method for stabilizing gelatinous photographic emulsions against fogging. A further object of this invention is to maintain the sensitivity and fog of silver halide gelatin emulsions at or close to initial optimum values under keeping and conditions of high temperature and humidity. A further object is to provide photographic silver halide gelatin emulsions containing antifoggants or stabilizers. A further object is to provide an anti-foggant which Will permit a prolonged digestion of the gelatin emulsion without any danger of premature fogging and at the same time prevent any change in emulsion characteristic during its preparation. Other objects will become apparent from a consideration of the following description and examples.

According to certain aspects of this invention, photographic gelatin emuisions may be protected against chemical fog by closely associating with the emulsion, a stabilizing or anti-fogging agent selected from the group consisting of 6-thioctic acid, amine salts of 6-thi0ctic acid, alkali metal salts of 6-thioctic acid, and alkaline earth metal salts of 6-thioctic acid. In general, 6-thioctic acid can be prepared according to the method described by E. A. Brande et al., Chemistry and Industry, April 30, 1955, page 508. The salts of the acid can be prepared by substitution reaction. The method described consists of preparing hept-6-enoic acid (II) from either tetra-hydrofurfuryl alcohol or tetra-hydrofuran. Reaction of hept-6-enoic acid (H) with formaldehyde in a mixture of acetic and sulfuric acids gives the dioxan (III; R=H) which is converted by diazomethane into the methyl ester (III; R=Me) and thence by methanol-sulfuric acid into methyl 6:8-dihydroxyoctanate (IV). Reaction of the latter with thiourea and hydriodic acid followed by alkaline hydrolysis provides the dithiol (V; R=H) which on oxidation with iodine furnishes thioctic acid (I). This product (I) is referred to as 6-thioctic acid or as 6-8-thioctic acid.

CH2:CH.[CI-I2]4.CO2 (IJHLOHLCHJCHQLEOZR The above example will serve to illustrate a method by which the stabilizer of this invention may be prepared, but is not to be construed as limiting the invention.

Although 6-thioctic acid itself can be used as the antifogging agent, it is preferred to use salts thereof. It is found that particularly satisfactory results are obtained when amine salts of the acid are used. Typical of these are the salts of the acid with amines such as methylamine, ethyl-amine, butylamine or morpholine; olamines such as monoethanolamine, diethanolamine, or triethanolamine can also be used.

Other compounds found to be particularly effective are the alkali metal salts of 6-thioctic acid e.g. the sodium or ammonium salts, or the alkaline earth metal salts e.g. the barium, strontium, or calcium salts.

The preparation of silver halide emulsions includes three separate operations: (1) the precipitation and digestion or ripening of the silver halide, (2) the freeing of the emulsion from excess soluble salts, usually by washing, and (3) the second digestion or after-ripening to obtain increased sensitivity.

The photographic emulsions used in practicing our invention are generally of the ammonia or boiled type; also it is to be understood that photographic emulsions of varying halide content can advantageously be used. The antifoggant compounds used in our invention have been found particularly useful when employed in conjunction with gelatino-silvcr bromiodide emulsions, although they can also be advantageously employed for stabilizing other silver halide emulsions, such as gelatinosilver chloride, bromide, chlorobromide, chlorobromiodide, etc. The emulsion can be unsensitized, or sensitized With chemical sensitizers or optical sensitizers.

The fog-inhibiting agents useful in practicing our invention can be used in various kinds of photographic gelatin emulsions. In addition to being useful in ordinary non-sensitized emulsions, they can also be used in orthochromatic, panchromatic and X-ray emulsions. If used with sensitizing dyes, they can be added to the emulsion before or after the dyes are added. A suitable dispersing agent for the silver halide is gelatin which stabilizes the silver halides to provide a photographic emulsion.

In practice of the invention, according to certain of its aspects, the stabilizing or antifogging 6-thioctic acid or its derivative is added before the emulsion has set. Preferably, the 6-thioctic acid or its derivatives will be added after the final digestion or after ripening although it can be added prior to either digestion step.

Instead of adding the fog-inhibiting agent directly to the photographic emulsion, it is sometimes desirable to incorporate the fog-inhibiting agent in a separate layer which is placed in contact with the silver halide emulsion layer. Under such conditions, it is preferable to use a higher concentration of fog-inhibiting agent than indicated above e.g. typically ten times as much, or more.

It is found that when the hereinbefore noted antifogging or stabilizing agents are present in close association with the photographic silver halide emulsion, i.e. whether they are intimately admixed with the emulsion prior to setting or coated on the emulsion in a separate layer or top coating, the eminently satisfactory results of this invention are obtained.

Typically the 6-thioctic acid or its derivatives will be added to the emulsion as an aqueous solution of 0.1% to 0.4%, say 0.25% concentration by Weight.

When the thioctic acid derivative employed is a water soluble derivative it will be dissolved in water to form an aqueous solution. In the case of 6-thioctic acid itself, satisfactory results may be obtained by forming this compound in situ by dissolving the sodium salt in solution in the emulsion and lowering the pH to less than 7 whereby the 6-thioctic acid is formed and precipitates in the form of a colloidal suspension in the emulsion which is effective to produce the desired antifogging result. Preferably the fog inhibiting 6-thioctic acid or its derivatives will be employed in an amount which may be 0.005 mg.10 mg. per gram of gelatin. When the 6-thioctic acid or its derivatives is present directly in the emulsion, the concentration thereof will be 0.005 mg.0.05 mg. per gram of silver halide.

Although the improved results of this invention may be obtained by adding the 6-thioctic acid salts to the emulsion before it is coated onto the support, preferably it is added after the final digestion or ripening. When the emulsion has been coated or dried on the carrier, the advantages of this invention can be obtained by depositing the 6-thioctic acid salts as a separate coating or layer by forming an aqueous solution thereof of 0.0017% to 0.017%, say 0.01% concentration. To this solution may be added 3% to say 4% of gelatin on top of the layer of emulsion.

When the fog inhibiting agent is present in the film outside of the emulsion as either i.e. a top coating or an undercoating, it preferably will be present in an amount which may be O.5-10 mg. per gram of gelatin. Preferably the concentration of additive Will be 0.52.5 mg. per gram of gelatin when the additive is present in the top coating; and 2-10 mg. per gram of gelatin when the additive is present in the undercoating.

The fog inhibitors which We propose to use are added to avoid loss of sensitivity and to inhibit the growth of fog with passage of time under non-ideal conditions of storage. 6-thioctic acid or its derivatives permit prolonged digestion of the emulsion without any danger of running the emulsion into premature fog and it safely stops any change of emulsion characteristics during holding times prior to coating or during the time required for coating the entire emulsion. That is, thioctic acid or its derivatives preserve the emulsion characteristics perfectly from the beginning to the end of coating. It is, therefore, a very good stabilizer of the emulsion in the wet state or, a powerful restrainer or retarder of changes during the handling of the emulsion in the wet state.

A solution of the compounds of the invention when added in suitable concentration, before coating, to unsensitized, chemically sensitized, or optically sensitized photographic emulsions does appreciably stabilize photographic speed and maintain fog at a low level. When sensitometric measurements are made at appreciable intervals of time, at elevated temperatures and dry or somewhat humid conditions, these compounds continue to stabilize photographic speed and maintain fog at a low level.

6-th-ioctic acid and its salts not only retard the advent of fog, but in some instances even permit the making of fast and vigorous emulsions without fog which in the absence of thioctic acid cannot be digested into speed or gradation without fog formation.

In order to evaluate the advantages which occur from the use of this invention, an emulsion prepared according to this invention will be compared with a standard emulsion. Although, as hereinbefore noted, this invention may be used in connection with any of a wide variety of types of emulsion, for purposes of evaluation, reference will hereinafter be made to a precipitated boiling emulsion prepared according to the procedure outlined below.

Preparation of the emulsion which will be used as a standard in the several examples hereinafter set forth will be as follows:

PREPARATION OF THE STANDARD PRECIPI- TATED BOILING EMULSION Solution 1:

Gelatin g 9.0 Potassium bromide g 80.5 Potassium iodide g 5.0 Distilled water up to cc 72 Solution II:

Silver nitrate g Distilled water up to cc 800 Solutions I and II are each separately prepared by dissolving the several solid ingredients in each, in distilled water and adding additional distilled water up to the volumes indicated. The solutions are then adjusted to a temperature of 70 C.

300 cc. of solution II at 70 C. are added to 725 cc. of solution I, the mixture being continuously stirred. The reaction of the components of solutions I and II results in precipitated silver halide. 30 seconds after that addition is complete, addition of the remainder of solution H is started; 100 cc. being added at that time and the remaining portion of solution II added in 100 cc. aliquots at 10 minute intervals until all of solution II has been added to solution I.

The completed emulsion so formed is digested for 20 minutes at 70 C. after which time it is cooled to 38 C.;

40 cc. of a fresh solution of zirconyl sulfate are then added to the precipitated emulsion while stirring. The precipitate is allowed to settle and then stand for 5 minutes.

The supernatant liquor is poured ofi and the precipitated emulsion is vigorously washed twice with two 800 cc. portions of cold distilled H O within 5 minutes. The precipitated emulsion is subjected to four further washings with distilled water, each washing being effected by pouring off the supernatant liquor, adding a fresh 800' cc. aliquot of cold distilled H O, stirring vigorously for 10 minutes, allowing the precipitate to settle and again pouring off the supernatant liquor.

The washed precipitated emulsion is then soaked for 1 hour in 700 cc. of distilled H O containing 2 g. of dissolved sodium bicarbonate with vigorous stirring. The supernatant liquor is decanted and the precipitated emulsion is soaked for 30 minutes in 100 cc. of distilled H O containing 3 g. of dissolved sodium citrate with vigorous stirring.

The precipitated emulsion is then heated while in intimate contact with the sodium citrate solution to 55 C. while stirring to effect solution of the precipitated emulsion. To this heated solution is added 1100 cc. of distilled water at 20 C. and the two are mixed thoroughly. Pour 200 cc. of the finished emulsion onto 20 g. of solid gelatin and allow to stand for minutes. Heat the mixture to 50 C. to effect solution of the gelatin in the emulsion.

To this solution is added 0.5 cc. of an ammonium gold thiocyanate solution (prepared by adding 6 cc. of 1% gold chloride, HAuCl4.3H O, to 5 cc. of 1% ammonium thiocyanate solution) and letting the solution stand until it is clear. The emulsion is then digested at 60 C. until the maximum speed just prior to fog development is reached by digesting into fog and ripening to pH 7.

The following examples will illustrate specific embodiments of this invention and by comparison with the hereinbefore defined standard emulsion, will indicate the improvement obtained by the instant invention.

Example I The procedure for preparing the above-noted standard emulsion was followed in every detail, except that 1.0 cc. of 0.40% of the sodium salt of 6-thioctic acid was added simultaneously with the 0.5 cc. of the ammonium gold thiocyanate solution.

Example II The procedure for preparing the above-noted standard emulsion was followed in every detail except that 0.5 cc. of 0.5% of the ammonium salt of 6-thioctic acid was added simultaneously with the 0.5 cc. of the ammonium gold thiocyanate solution.

Example III The procedure for preparing the above-noted standard emulsion was followed in every detail except that 1 cc. of 0.2% of the mono-ethanolamine salt of 6-thioctic acid was added simultaneously with the 0.5 cc. of the ammonium gold thiocyanate solution.

Example IV The procedure for preparing the above-noted standard emulsion was followed in every detail except that 1 cc. of 0.3% of the methylamine salt of 6-t-hioctic acid was added simultaneously with the 0.5 cc. of the ammonium gold thiocyanate solution.

Example V The procedure for preparing the above-noted standard emulsion was followed in every detail except that 1 cc. of a saturated solution of the calcium salt of 6-thioctic acid was added simultaneously with the 0.5 cc. of the ammonium gold thiocyanate solution.

A comparative evaluation of this invention against the standard emulsion hereinbefore defined was made by forming a photographic pla-.e by depositing each of the emulsions on a suitable support, and then subjecting the same to a temperature of 55 C. and relative humidity of 60% for 5 days. At the conclusion of the test period, the fog density of the incubated, enfogged, developed and fixed emulsion was determined by standard techniques.

As is well known to those skilled in the art, untreated emulsions will normally have a fog density value of 0.30- 0.50. Fog densities less than 0.30, say down to 0.15 or less, are considered superior.

TABLE Emulsion: Fog density Unstabilized 0.4

tabilized:

Sodium 6-thioctate 0.15 Ammonium 6-thioctate 0.12 6-thioctic acid, monoethanolamine salt 0.08 6-thioctic acid, methylamine salt 0.09 Calcium 6-thioctate 0.10

It is apparent from inspection of the above table that the emulsions stabilized in accordance with this invention are characterized by a desirably low fog density of 0.15 or less.

The desirable results obtained with the precipitated boiling emulsion may also be obtained with other emulsions, e.g., precipitated ammonium emulsion, precipitated photostat emulsion, etc. by similarly treating these emulsions with the fog inhibitors of this invention.

Although this invention has been described with respect to certain specific embodiments thereof, it will be apparent to those skilled in the art that various modifications and changes may be made therein.

This application is a continuation-in-part of application Serial Number 681,882, filed September 4, 1957 by the same inventors.

What is claimed is:

1. A photographic gelatin silver halide emulsion in close association with an antifogging agent selected from the group consisting of amine salts of 6-thioctic acid, alkali metal salts of 6-thioctic acid, and alkaline earth metal salts of 6-thioctic acid, said agent being present in fog-inhibiting amounts.

2. A photographic gelatin silver halide emulsion in close association with an antifogging agent selected from the group consisting of amine salts of 6-thioctic acid, alkali metal salts of 6-thioctic acid, and alkaline earth metal salts of 6-thioctic acid, said agent being present in amount of 0.005 to 10 milligrams per gram of gelatin.

3. A photographic gelatin silver halide emulsion hearing a coating comprising an antifogging agent selected from the group consisting of amine salts of 6-thioctic acid, alkali metal salts of G-thioctic acid, and alkaline earth metal salts of 6-thioctic acid, said agent being present in amount of 0.5 to 10 milligrams per gram of gelatin in the emulsion.

4. A photographic gelatin silver halide emulsion in close association with ammonium 6-thioctate, said ammonium thioctate being present in amount of approximately 0.005 to 10 milligrams per gram of gelatin in the emulsion.

5. A photographic gelatin silver halide emulsion having contained therein an antifogging agent selected from the group consisting of amine salts of 6-thioctic acid, alkali-metal salts of 6-thioctic acid, and alkaline earth metal salts of 6-thioctic acid in amount of 0.005-0.05 milligram per gram of gelatin in the emulsion.

6. A photographic gelatin silver halide emulsion coated with a top coating selected from the group consisting of amine salts of 6-thioctic acid, alkali metal salts of 6- thioctic acid, and alkaline earth metal salts of 6-thioctic acid in amount of 0.5-2.5 milligrams per gram of gelatin in the top coating.

7. The method of decreasing the susceptibility to fogging of a photographic gelatin-silver halide emulsion which in untreated form is characterized by a high susceptibility to fogging which comprises treating said photographic gelatin silver halide emulsion with fog-inhibiting amounts of an anti-fogging agent selected from the group consisting of amine salts of 6-thioctic acid, alkali metal salts of 6-thioctic acid, and alkaline earth metal salts of 6- thioctic acid.

8. The method of decreasing the susceptibility to fogging of a photographic gelatin silver halide emulsion which in untreated form is characterized by a high susceptibility to fogging which comprises treating said gelatin silver halide emulsion with an anti-fogging agent selected from the group consisting of amine salts of 6-thioctic acid, alkali metal salts of 6-thioctic acid, and alkaline earth metal salts of 6-thioctic acid, said agent being present in amount of 0.005 to 10 milligrams per gram of gelatin.

References Cited in the file of this patent UNITED STATES PATENTS 2,728,668 Mochel Dec. 27, 1955 2,756,145 Ballard et a1 July 24, 1956 2,948,614 Allen et al Aug. 9, 1960 

1. A PHOTOGRAPHIC GELATING SILVER HALIDE EMULSION IN CLOSE ASSOCIATION WITH AN ANTIFOGGING AGENT SELECTED FROM THE GROUP CONSISTING OF AMINE SALTS OF 6-THIOCTIC ACID, ALKALI METAL SALTS OF 6-THIOCTIC ACID, AND ALKALINE EARTH METAL SALTS OF 6-THIOCTIC ACID, SAID AGENT BEING PRESENT IN FOG-INHIBITING AMOUNTS. 